1. Field of the Invention
The present invention relates to a superconducting logic circuit, which utilizes Josephson junctions so that it can be driven by a d.c. power source, and a switching device for use therewith. The circuit of the present invention is of the type, which is completely symmetric with respect to the grounded point and in which a signal current is injected directly into a switching device. The logic circuit of the present invention can be used as an integrated circuit for composing a high speed computer.
2. Description of the Prior Art
Most of the circuits utilizing Josephson junctions make the existence of a current signal correspond to logic values "1" and "0" but can make positive and negative currents correspond to the values "1" and "0" in accordance with the direction of the current. As one of those circuits, there has been known in the prior art a HUFFLE (an abbreviation of Hybrid Unlatching Flip-Flop Logic Elements) circuit (which should be referred to: A. E. Hebard, et al. "ADC-Powered Josephson flip-flop", IEEE Trans. MAG-15, [1]408(1979); T. A. Fulton, et al, "Josephson junction current-switched logic circuits", IEEE Trans. MAG-15, [6]1876(1979); and S. S. Pei, "Current-switched Josephson flip-flop logic", 1979-MTT-S, International Microwave Symposium Digest, 21). This HUFFLE circuit is driven by a current source and a current sink, as shown in FIG. 1. The HUFFLE circuit is featured by the fact that those power sources may be of d.c. type. With the respective power source terminals, there are connected two superconducting switching devices which are indicated at SW and which have their other terminals grounded. With those power source terminals, moreover, there are connected two identical resistors R which have their other terminals connected with each other to provide an output terminal. As a result, a positive or negative output current I.sub.out is generated at an inductive load (which has an inductance L) in response to the input signals of the switching devices which are located at the righthand and lefthand sides of the drawing. This drawing shows an example, in which one of the simplest Josephson junctions (which are indicated at X in the drawing) is used as the switching device, as a representative. At this time, the input signal currents flow through the control lines, which are indicated at broken lines in the drawing, so that magnetic fluxes are generated to exert influences upon the operations of the junctions. Now, if the lefthand junction is fed with an input so that it is transferred from a superconducting state (i.e., a 0 voltage state) to a voltage state, the current I.sub.o of the lefthand current source cannot flow directly to the grounded point but flows through the load inductance L so that the positive output current I.sub.out is derived. Since, at this time, the righthand junction is at its superconducting state, the current of the righthand current source flows as it is from the grounded point to the power source but does not appear at the output terminal. At a next step, if the righthand junction is fed with an input so that it is transferred to a voltage state, the opposite state to the above comes into effect. As a result, the current of the sink source flows through the load inductance L so that the negative output current I.sub.out is derived. This fact that the lefthand junction is returned to its voltage state at the instant of the transfer is the most fundamental feature of the operations of the HUFFLE circuit. As has been briefly described in the above, the known HUFFLE circuit has the two separate input terminals so that it can generate, when either (not both) of its input terminals receives an input signal, an output signal corresponding to the input signal, and can maintain that state even after the input disappears. As a result, the HUFFLE circuit can be used as a flip-flop circuit. In FIG. 1, there are shown parasitic inductances L.sub.1 ' and L.sub.2 ' which accompany the junction circuit but may be essentially considered not to exist (or to be zero). In case the HUFFLE circuit thus far described is used as a logic gate, there arises a following problem. In the aforementioned example, if the first input is fed to the lefthand switching device, a subsequent second input has to be impressed upon the righthand switching device after lapse of a certain time. If the second input is impressed upon the lefthand switching device, the circuit has a defect that it does not respond. In order to eliminate this defect, complementary inputs may be always (or simultaneously) impressed upon the righthand and lefthand input terminals. More specifically, it is sufficient that a logic value "1" (or "0") is never fed to the lefthand input terminal but a logic value "0" (or "1") is simultaneously fed to the righthand input terminal. From the standpoint of the construction of a logic circuit, however, it is highly wastefull to prepare the aforementioned "complementary input signals" at all times.
In the known example thus far described, it is known that a superconductive switch called "JAWS" (an abbreviation of Josephson Atto-Weber Switches), as shown in FIG. 2, is used in place of a mere junction as the switching device. (Reference should be made to: T. S. Stakelon, "Current switched Josephson latching logic gates with sub-100 ps delays", IEEE Trans. MAG-15, [6]1886(1979).) The operation of that switch is featured by that an input current I.sub.in is injected directly into an active portion but not by that an input is received in the form of a magnetic flux, as in the aforementioned junction switch. The HUFFLE circuit utilizing the JAWS as the switching device is called "JAWS-HUFFLE" and is shown in FIG. 3. According to the References cited hereinbefore, any one can be used as the "switching device" if it can take as a whole the two states, i.e., the super-conducting state (i.e., the zero voltage state) and the voltage state and if the current to flow through the switching device is sufficiently smaller than the current in the superconducting state. Therefore, the HUFFLE circuit can be constructed by using the CIL gate disclosed in U.S. Pat. No. 4,117,503 as the switching device.
Within a range coming from the aforementioned examples or to be easily deduced therefrom, it is apparent that the HUFFLE circuit cannot be operated without preparing the "complementary input signals". In the aforementioned reference of T. A. Fulton, there is found a description that the HUFFLE circuit can be operated with one input by connecting two input terminals I.sub.in and I.sub.in ' by means of a resistor R', as indicated by broken line in FIG. 3, instead of preparing those "complementary input sginals". However, it is not easy to design the circuit operating point therefor. Incidentally, all of the prior art examples thus far described can be considered, if summarized, to be a kind of the "bridge type logic circuit", in which the two superconducting switching devices SW and the two resistors are connected in the form of a Wheatstone bridge, as shown in FIG. 4.